The present disclosure relates to delta-sigma analog-to-digital (A/D) converters, and more particularly, to a resonant filter which can switch between filtering operation and oscillating operation.
In continuous-time delta-sigma A/D converters, an RC active filter including a resonant filter is often employed as the loop filter. However, the resistance value and the capacitance value of a resistor and a capacitor (hereinafter referred to as “the values of a resistor and a capacitor”) which are included in the RC active filter vary due to manufacturing variations etc., and therefore, the filter characteristics of the RC active filter vary. The variations in filter characteristics cause a deterioration in the stability and signal-to-noise (S/N) ratio characteristics of the A/D converter.
To address the above problem, in general, for example, the frequency characteristics etc. of the RC active filter are directly measured using a measuring instrument to adjust the values of a resistor and a capacitor included in the RC active filter, in a manufacturing process, a testing process, etc. Specifically, for example, a delta-sigma A/D converter and an RC oscillator may be included together, the product of R and C may be obtained from the oscillation frequency of the RC oscillator, and based on the obtained RC product, the values of a resistor and a capacitor included in the RC active filter may be adjusted (see Kazuo Matsukawa, et al., “A 69.8 dB SNDR 3rd-order Continuous Time Delta-Sigma Modulator with an Ultimate Low Power Tuning System for a Worldwide Digital TV-Receiver,” Custom Integrated Circuits Conference (CICC), 2010 IEEE (USA), 19-22 Sep. 2010, p. 1-4).
FIG. 12 is a diagram showing an example circuit configuration of a circuit in which a delta-sigma A/D converter and an RC oscillator are included together.
The delta-sigma A/D converter 900 includes a loop filter 901, a quantizer 904, and current-drive feedback D/A converters 905 and 906.
The loop filter 901 includes an integrator 902, and a resonant filter 903 in a stage following the integrator 902. The resonant filter 903 outputs differential output signals to the quantizer 904 through a resistor R93. The loop filter 901 feeds back the differential output signals to the respective corresponding differential inputs of the integrator 902 and the resonant filter 903 through respective corresponding resistors R91 and R92.
The differential signals input to the quantizer 904 are converted into a digital signal. The digital signal obtained by the conversion is fed back through the feedback D/A converters 905 and 906 as analog signals to the respective corresponding inputs of the quantizer 904, and the loop filter 901.
An RC oscillator 907 outputs an oscillating signal which oscillates at a predetermined oscillation frequency. A frequency measurement circuit 908 receives the oscillating signal from the RC oscillator 907, and measures a difference between the frequency or period of the oscillating signal and a preset oscillation frequency. A controller 909 obtains, based on the frequency or period difference received from the frequency measurement circuit 908, a deviation of the values of the resistor and the capacitor due to manufacturing variations etc., and outputs an RC tuning signal SC9 for adjusting the values of the resistor and the capacitor to the integrator 902 and the resonant filter 903. As a result, the deviation of the values of the resistor and the capacitor due to manufacturing variations etc. is adjusted (corrected).